1. Field of the Invention
The present invention relates to a microwave oscillator having a diode stabilized by a dielectric resonator, this oscillator being endowed with enhanced stability in comparison with oscillators of known types.
This oscillator operates in the W band between 56 and 100 GHz and its design has been guided by the need to provide high stability in order to permit alternate operation in transmission and in reception, with a very short time of recovery of its natural oscillation frequency after each transmission-reception changeover.
2. Description of the Prior Art
Occupancy of the frequency spectrum in the I and J bands, respectively from 8 to 10 GHz and from 10 to 20 GHz, has led to an endeavor to use higher frequencies at millimeter wavelengths, in particular in the field of telecommunications.
The millimeter-wave region of the spectrum offers an advantage in that it achieves higher orientation resolution as well as better penetration of the atmosphere and of clouds. Thus the choice of 94 GHz is justified by the existence of an atmospheric propagation window in the vicinity of this frequency.
Communication by means of microwaves relies on the design of an oscillator having very good stability, good noise characteristics in amplitude modulation and medium power, while being capable of performing the dual function of local oscillator and transmission source in a transmitter-receiver system which operates in the millimeter-wave band. This design permits of two alternatives:
direct generation, PA0 mixing of a low-noise stable millimeter-wave oscillator and a lower-frequency voltage-controlled oscillator.
The problems of transition from the transmission state to the reception state are in this case transposed to a lower frequency and are therefore reduced.
In known oscillators of the negative-resistance diode type stabilized by a dielectric resonator, this latter plays the part of a short-circuit which is placed as close as possible to the diode; the performances of the oscillator in accordance with this design are therefore equivalent to those of an all-metal resonant cavity of larger volume with which the diode would be magnetically coupled, the resonator being such as to correspond to one of the walls of the metal cavity. The volume of the cavity thus created is smaller, which explains the reduction in frequency drift obtained. The dielectric resonator does not represent the principal resonant element in this structure. In consequence, despite optimiation of the temperature coefficient of the resonator material, it appears difficult with a system of this type to achieve a degree of frequency stability which is compatible with the stability required by transmitter-receiver systems.
In the oscillator in accordance with the invention, the dielectric resonator is the only resonant element, thus ensuring higher frequency stability and higher purity of the spectrum. The diode is positioned with respect to the waveguide in such a manner as to ensure that the oscillation frequency corresponds to the natural resonance frequency of the resonator; this latter is placed with respect to the diode so as to provide an optimum diode-resonator-waveguide coupling. This coupling is obtained by means of a coupling loop constituted by a capacitor mounted between the resonator and the metallic cap of the diode package and by a wire connected between the top plate of the capacitor and ground.
Biasing of the diode is performed in the plane of the waveguide, thus achieving enhanced compactness of the system. Matching of the oscillator with the useful load (optimum power) is caried out by means of variable irises and by vertical positioning of the diode support since this mode of assembly permits positional variation.
In respect of a frequency f.noteq.fo (natural frequency of the resonator), the resonator behaves as a short-circuit and the diode is loaded by the auxiliary load. No oscillation appears.
When f=fo, the resonator has an infinite load and adjustment of its position with respect to the diode makes it possible to restore the correct impedance in this latter so as to induce oscillation. Accordingly, the dielectric resonator is the only resonant element which establishes the frequency of the oscillator.